As a working roll for hot rolling, a centrifugally cast composite roll comprising a centrifugal cast outer layer made of a wear-resistant iron-based alloy, and an inner layer made of tough ductile cast iron and integrally fused to the outer layer is widely used. Because the outer layer of the centrifugally cast composite roll is subjected to a thermal and mechanical load by contact with a strip to be rolled, its surface suffers damages such as wearing, roughening, etc., damaging the surface quality of a rolled strip. Accordingly, when the outer layer suffers damages to some extent, the roll is exchanged. After damaged portions are removed by machining from an outer layer of a roll taken out of a mill, the roll is assembled again in the mill. The removal of damages from an outer layer of a roll is called “damage-removing cutting.” The working roll is discarded, after it is cut to remove damages from the initial diameter to the minimum diameter usable for rolling (discard diameter). A diameter in a range from the initial diameter to the discard diameter is called “an effective rolling diameter.” Because frequent damage-removing cutting lowers productivity by the interruption of rolling, an outer layer in an effective rolling diameter desirably has excellent wear resistance, failure resistance and surface roughening resistance to avoid large damages.
As shown in FIG. 1, a composite roll 10 comprises an outer layer 1 coming into contact with a strip to be rolled, and an inner layer 2 fused to an inner surface of the outer layer 1. The inner layer 2 is made of a material different from that of the outer layer 1, and constituted by a core portion 21 fused to the outer layer 1, and a drive-side shaft portion 22 and a free-side shaft portion 23 integrally extending from both sides of the core portion 21. An end portion of the drive-side shaft portion 22 is integrally provided with a clutch portion 24 for transmitting a driving torque. An end portion of the free-side shaft portion 23 is integrally provided with a projection 25 necessary for handling the composite roll 10. The clutch portion 24 has an end surface 24a, and a pair of flat notch surfaces 24b, 24b engageable with a driving means (not shown). The projection 25 has an end surface 25a. The drive-side shaft portion 22 and the free-side shaft portion 23 should be machined to form bearing portions, neck portions, etc.
When the composite roll 10 is used for hot rolling, damages such as wearing, seizure, surface roughening, etc. should be suppressed in the outer layer 1, and wearing should also be suppressed in the clutch portion 24 of the drive-side shaft portion 22 connected to a coupling means for torque transmission, to expand damage-removing cutting intervals as much as possible. To this end, it is important that the outer layer 1 is made of an iron-based alloy having excellent wear resistance and failure resistance, and that the inner layer 2 is made of ductile cast iron having improved wear resistance.
As a hot-rolling composite roll having a clutch portion with improved wear resistance, JP 6-304612 A discloses a hot-rolling composite roll comprising an outer layer made of high-speed tool steel, an inner layer and shaft portions both made of carbon steel or low-alloy steel containing 0.2-1.2% by weight of C, and a clutch portion connected to an end of a shaft portion by casting, the clutch portion being made of spheroidal graphite cast iron comprising by weight 2.5-3.5% of C, 1.6-2.8% of Si, 0.3-0.6% of Mn, less than 0.05% of P, less than 0.03% of S, less than 0.5% of Ni, less than 0.2% of Cr, less than 0.5% of Mo, and 0.02-0.05% of Mg, the balance being Fe and other inevitable components, in which an area ratio of graphite is 5-15%. However, this clutch portion has still insufficient wear resistance. Because the clutch portion is connected to an end of the shaft portion, their connection interface likely suffers casting defects such as foreign matter contained, etc. Further, it suffers a high production cost, because it needs machining a connected-by-casting portion flatly, using a mold surrounding a portion to be connected by casting, and casting spheroidal graphite cast iron for the clutch portion different from the inner layer.
JP 2012-213780 A discloses a rolling composite roll comprising an outer layer, a core portion formed inside the outer layer, a drive-side shaft portion extending from one end of the core portion, and a free-side shaft portion extending from another end of the core portion, the drive-side shaft portion having higher tensile strength than that of the free-side shaft portion by 50 MPa or more. As a method for providing different tensile strength to the drive-side shaft portion and the free-side shaft portion, JP 2012-213780 A describes a stationary casting method with a drive-side shaft portion on the lower side and a free-side shaft portion on the upper side, using a die coated with a refractory material for a lower mold for the drive-side shaft portion, and a sand mold for an upper mold for the free-side shaft portion, the casting temperature being lower in the lower mold than in the upper mold by 20° C. or more. However, even though the casting temperature difference can make the drive-side shaft portion stronger than the free-side shaft portion, it fails to provide the drive-side shaft portion higher wear resistance than that of the free-side shaft portion.
In addition, if the entire inner layer 1 were made harder to provide the drive-side shaft portion 22 with increased wear loss resistance, the free-side shaft portion 23 would also become harder, resulting in lower machinability.